TECHNIQUES OF POLYMERIZATON The following methods are generally used for the polymerisation reaction

1. Bulk polymerisation.

2. Solution polymerisation.

3. Suspension polymerisation.

4. Emulsion polymerisation

1. Bulk Polymerisation

Bulk polymerisation is the simplest method of polymerisation. The monomer is taken in a flask as a liquid form and the initiator, chain transfer agents are dissolved in it. The flask is placed in a thermostat under constant agitation and heated.

The reaction is slow but becomes fast as the temperature rises. After a known period of time, the whole content is poured into a methanol (non-solvent) and the polymer is precipitated out.

Polystyrene, PVC, PMA are preparedby this method

Advantages

1. It is quite simple and requires simple equipments.

2. Polymers are of high-purity obtained.

3. As the monomer is solvent, excess monomer can

be removed by evaporation.

4. The polymer has high optical clarity.

Disadvantages

1. During polymerisation, viscosity of the medium increases

hence mixing and control of heat is difficult.

2. Polymerisation is highly exothermic

Applications

1. The polymers obtained by this method are used in

casting formulations.

2. Low molecular weight polymers, obtained by this

method, are used as adhesives, plasticizers and lubricant

additives

2. Solution Polymerisation

In solution polymerisation, the monomer, initiator and the chain transfer agents are taken in a flask and dissolved in an inert solvent. The whole mixture is kept under constant agitation. After required time, the polymer produced is precipitated by pouring it in a suitable non- solvent.

The solvent helps to control heat and reduces viscosity built

up.

Polyacrylic acid, polyisobutylene and

polyacrylonitrile are prepared by this method.

Advantages

1. Heat control is easy.

2. Viscosity built up is negligible.

3. The mixture can be agitated easily

Disadvantages

1. The removal of last traces of solvent is difficult.

2. This polymerization requires solvent recovery and recycling.

3. It is difficult to get very high molecular weight polymer.

4. The polymer formed must be isolated from the solution either by evaporation of the solvent or by precipitation in a non-solvent.

Applications

As the polymer is in solution form, it can be directly used

as adhesives and coatings.

3. Suspension Polymerisation

Suspension polymerisation is used only for water insoluble

monomers.

This polymerisation reaction is carried out in heterogeneous

systems.

At the end of polymerisation, polymer is separated out as

spherical beads or pearls.

This method is also called pearl polymerisation.

The water insoluble monomer is suspended in water tiny droplet and a initiator is dissolved in it by continuous agitation.

The suspension (droplets) is prevented from coagulation by using suspending agents like PVA, gelatin, methyl cellulose. Each droplet of the monomer contains dissolved initiator.

The whole content is taken in a flask and heated at constant temperature with vigorous agitation in a thermostat with nitrogen atmosphere. After the end of 8 hrs, pearl-like polymers are obtained, which is filtered and washed by water.

Polystyrene, Polystyrene-divinyl benzene

Advantages1. Since water is used as a solvent, this method is more economical.

2. Products obtained is highly pure.

3. Isolation of product is very easy.

4. Efficient heat control.

5. Viscosity build up of polymer is negligible.

Disadvantages

1. This method is applicable only for water insoluble

monomers.

2. Control of particle size is difficult.

Applications

1. Polystyrene beads are used as ion exchangers.

2. This technique is used in heterogeneous system.

4. Emulsion Polymerisation

Advantages

1. The rate of polymerisation is high.

2. Heat can be easily controlled and hence viscosity built up is low.

3. High molecular weight polymer can be obtained

Disadvantages

1. Polymer needs purification.

2. It is very difficult to remove entrapped emulsifier and de-emulsifiers.

3. It requires rapid agitation.

Applications

1. Emulsion polymerisation is used in large- scale production like water-based paints, adhesives, plastics, etc.,

2. This method is also suitable for manufacturing tacky polymers like butadiene and chloroprene.